The invention relates to a thermal printer with liftable printing head.
Hitherto thermal printers have been known, especially for use in fax apparatuses, with which a one-line constructed printing mechanism imprints a thermosensitive paper.
Differently thick printing media, however, cannot be imprinted with such a printing mechanism. There are known, further, printing mechanisms for label printing, with which a label course is printed in the continuous process. Here, too, it is known to use a one-line printing head which lies opposite a corresponding counter-pressure roller, in which the medium to be printed is led through the roller gap between the printing head and the counter-pressure roller. With such printing mechanisms for thermal printers there is present, however, the disadvantage that it is possible to imprint only a relatively constant thickness of the label courses. If thicker label courses are to be printed, this leads to high wear on the printing head, because the printing head lies on the oppositely lying counter-pressure roller.
With such printing mechanisms there is present, moreover, the disadvantage that a considerable slippage occurs in the first introduction of the label course into the printing mechanism, because the label course is brought up onto the printing head under corresponding friction manifestations from the counter-pressure roller. There arises thereby a considerable abrasion on the printing head, which leads to a premature wearing-out and to high costs associated with the replacement.
It is desired to be able to print labels of different thicknesses. Especially in the case of sales labels it is required that relatively thick labels be printed, because such sales labels can have integrated with them so-called transponder coils or other theft safeguards, which lead to a considerable build-up of the label height.
Underlying the invention is the problem of developing a printing mechanism for a thermal printer of the type mentioned at the outset, in such manner that with simple means differently thick printing media, especially label courses, can be printed, with which the wear on the printing head is to be minimized.
For the solution of the problem posed the invention is characterized in that the pressure beam of the printing head is constructed liftable (i.e., moved in a direction away from) and lowerable (i.e., moved in a direction toward) with the oppositely lying transport roller.
With the given technical teaching there is yielded the essential advantage that now a variable feed gap is provided between the printing head constructed as a pressure beam and the counter-pressure roller.
This counter-pressure roller is referred to in the following also as a transport roller, for it carries out the transport movement proper. Here the transport roller presses the medium to be printed, in particular the label courses, onto the underside of the pressure beam, which according to the invention is constructed liftable and lowerable.
In a preferred form of execution of the invention, the pressure beam is constructed as a lever swingable on one side.
Here it is preferred if the lifting occurs by a mechanical lifting element and the lowering occurs automatically, namely under weight load or by a spring or another contact-pressure element.
With the given technical teaching there is yielded, therefore, the substantial advantage that now a variable feed gap is provided between the transport roller and the free swingable end of the pressure beam, because this feed gap is enlarged in part by a corresponding lifting element andxe2x80x94depending on the position of this elementxe2x80x94is again reduced.
In a preferred form of execution of the invention the mentioned lifting element is constructed as a rotatably driven eccentric disk which is provided with a drive of its own allocated to this eccentric disk.
Here it is preferred if the eccentric disk is turnably borne on the same axis as the transport roller, but is driven independently from the transport roller.
Such a drive occurs, for example, over gear wheels which are suited to drive the eccentric disk separately from the transport roller.
For the infeed of a label course it is preferred, therefore, if the eccentric disk is first brought into a printing position with a cam projecting radially over the outer circumference, so that the infeed gap between the free swingable end of the pressure beam and the oppositely lying transport roller is kept as large as possible. Then a label course of arbitrary thickness is introduced into this infeed gap over a forward-thrusting drive (not further described) and, as soon as this infeed movement is ended, the rotation drive of the eccentric disk is again acted on, which is therewith turned into its minimal position, so that, conditioned by the contact-pressure element, the pressure beam now comes into opposition position to the transport roller and herewith a firmly defined infeed gap is given.
The medium to be imprinted now lies at a defined distance, between the printing head on the one hand, and the transport roller in the infeed gap on the other.
There can now take place a continuous printing operation, in which preferably the printing head always partially imprints the medium to be imprinted. After each printed line the transport roller is advanced by a further intermittent step, so that the next line of the printing format is reached and the one-line printing head now brings on the next printing line.
Instead of a thermosensitive printing head there can also be used other, one-line or optionally also multi-line printing heads, such as, for example, ink-jet printing heads, exposure printing heads that act on a corresponding exposure-capable printing medium.
With the given technical teaching, namely the use of a separately driven, turnable eccentric disk which is turnably arranged on the axis of rotation of the transport roller, there is yielded a substantial advantage over other liftable and lowerable arrangements. By reason of the design of the eccentric disk there is yielded, namely, a gentle transition between the lifting position and the lowered printing position, because, after all, the eccentric disk is constructed with a corresponding eccentric cam which brings about a corresponding sliding (and jolt-free) transition between the lifted and the lowered position of the pressure beam.
This is a substantial advantage over other lifting elements, such as, for example, an electromagnet or other abruptly acting lifting elements, because with these lifting elements there are still required additional shock absorbers and/or other measures have to be taken in order to bring about a gentle lowering of the pressure beam in the direction toward the transport roller.
Altogether, therefore, there is yielded with the proposal of a liftable printing head the substantial advantage that now arbitrarily thick media can be imprinted and that an especially gentle introducing of the print medium into the feed gap is possible, without there being present the danger that the pressure beam is damaged.
With the state of the art there is present, namely, the disadvantage that at present no defined minimal spacing is preset between pressure beam and transport roller. Therewith in every movement without printing material the roller rubs on the pressure beam. The eccentric disk offers a solution here. In the rotation of the eccentric disk the latter does not disappear behind the circumference of the transport roller, but reaches a position that brings about a minimum spacing.
The inventive object of the present invention is yielded not only from the object of the individual patent claims, but also from the combination of the individual patent claims among one another.
All the indications and features disclosed in the documents, inclusive of the abstract, especially the spatial design represented in the drawings, are claimed as essential to the invention insofar as, individually or in combination, they are novel with respect to the state of the art.
In the following the invention is explained in detail with the aid of drawings representing only one mode of execution. Here there proceed from the drawings and their description further features and advantages of the invention essential to the invention.